1. Field of the Invention
The present invention relates to a dielectric filter, a transmission-reception sharing unit, and a communication device for use in the microwave band and the millimeter-wave band.
2. Description of the Related Art
In order to achieve next-generation mobile and multimedia communications, ultra-fast transmission of a large amount of data is necessary. The millimeter-wave band having a wide bandwidth is suitable for this purpose. In addition, as another technology that can advantageously utilize the characteristics of the millimeter-wave band there is introduced collision-avoidance vehicle radar. Such millimeter-wave radar is greatly anticipated to improve safety in fog or snow. This is lacking in conventional laser radar using light.
When a conventional circuit structure mainly composed of microstrip lines is used in the millimeter-wave band, loss increases due to reduction of Q. A conventional TE.sub.01.delta. dielectric resonator, which is widely used, leaks a large amount of resonant energy out of the resonator. Thus, in the millimeter-wave band in which relative dimensions of the resonator and the circuit are small, the resonator undesirably couples with a line, thereby leading to difficulty in design and characteristic reproduction.
In order to overcome these problems, a millimeter-wave band module using the technology of PDIC.TM., which is a Planer Dielectric Integrated Circuit, may be mentioned. An example of a dielectric resonator incorporated in the module is shown in Japanese Unexamined Patent Application Publication No. 8-265015, the contents of which are included herein for reference.
In the above-mentioned dielectric resonator, electrodes formed on both main surfaces of a dielectric plate have openings in which the surfaces of the dielectric plate are exposed. The openings oppose to each other, so that the dielectric plate between the openings may act as a dielectric resonator.
FIGS. 7A, 7B, and 7C show an example of a dielectric filter using a plurality of resonators. FIG. 7A shows a view in which the upper conductor part of the dielectric filter is removed; FIG. 7B shows a sectional view taken along the line A--A in FIG. 7A; and FIG. 7C shows a sectional view taken along the line B--B in FIG. 7A. In this figure, reference numeral 3 denotes a dielectric plate; on a first main surface of which an electrode 1 is formed having electrodeles parts 4a and 4b; and on a second main surface of the plate, an electrode 2 is formed having electrodeless parts 5a and 5b opposing the electrodeless parts 4a and 4b. Parts of the dielectric plate positioned between these electrodeless parts operate as TE010-mode dielectric resonators. Coaxial connectors 10 and 11 are formed in a cavity 8, and probes 6 and 7 are protruded from the respective central conductors thereof so as to respectively couple with the dielectric resonator.
In the dielectric filter shown in FIGS. 7A, 7B, and 7C, spurious responses result in problems, as described below.
FIG. 8 shows attenuation characteristics of the dielectric filter shown in FIGS. 7A, 7B, and 7c. Responses of each mode are shown: reference character (a) indicates HE110 mode; reference character (b) indicates HE210 mode; reference character (c) indicates HE310 mode; reference character (d) indicates TE110 mode; and reference character (e) indicates TE010 mode. In addition to responses of the TE010 mode, which is a main mode, a number of unnecessary spurious responses occur. When these spurious responses coincide with frequencies in which specified attenuation levels are necessary, they may not satisfy required attenuation levels.
FIGS. 9A to 9E shows examples of electromagnetic field distributions of the above-indicated respective resonant modes. In these figures, solid lines indicate electric field, and broken lines indicate magnetic field. In each of the figures, the upper part shows a plan view of a dielectric resonator, and the lower part shows a view from the sectional direction of the dielectric plate.
FIGS. 9A to 9E show coupling states in each mode between two adjacent dielectric resonators. In any of the modes, magnetic-field coupling occurs between the adjacent dielectric resonators at the mutually near part.